High Number of Tornadoes This Year from Tropical Systems Nov 7, 2017

The 2017 hurricane season has been very active with 17 named storms and six making landfall in the United States. The tropical cyclones have also produced a lot of tornadoes, the most active season since 2008 and forth highest since 1995. The Storm Prediction Center issued a preliminary report today saying there has been 119 tornadoes associated with tropical systems this season.  Nearly half of the tornadoes this year were produced by Hurricane Harvey with 57. More than 300 tornado warnings were issued during Hurricane Harvey, the majority in Texas. Meteorologist Patrick Crawford has more in this video report on the 2017 season:

Tornadoes often occur in landfalling hurricanes, with conditions varying on each tropical system. Strength is not always a factor, as some tropical storms have been responsible for more tornadoes than major hurricanes. Environmental conditions, such as wind shear and instability, seem to relate more to tornado production during tropical systems rather than storm strength.

In tropical cyclones, the vertical structure of the atmosphere differs somewhat from what is most often seen in midlatitude low pressure systems. In particular, most of the thermal instability (heat and humidity) is found near or below 10,000 feet altitude, in contrast to midlatitude systems, where the instability maximizes typically above 20,000 feet. Because the instability in tropical cyclones is focused at lower altitudes, the storm cells tend to be smaller and shallower than those usually found in most severe midlatitude systems. But because the vertical shear in tropical cyclones is also very strong at low altitudes, the combination of instability and shear can become favorable for the production of small supercell storms, which have an enhanced likelihood of spawning tornadoes compared to ordinary thunderstorm cells.

Almost all tropical cyclones making landfall in the United States spawn at least one tornado, provided enough of the tropical cyclones circulation moves over land. This implies that Gulf Coast landfalling tropical cyclones are more likely to produce tornadoes than Atlantic coast tropical cyclones that sideswipe the coastline. The rate at which tropical cyclones produce waterspouts over the ocean is unknown, although Doppler radars have identified many cases where storm cell rotation suggestive of the presence of tornadoes was observed over water.  The right-front quadrant (relative to tropical cyclone motion) is the most favored for tornado development. Most of the tornadoes form in outer rainbands some 50-200 miles from the tropical cyclone center, but some have been documented in the inner core, or even in the eyewall. Tropical system can spawn tornadoes from a day or two prior to landfall to up to three days after landfall. Statistics show that most of the tornadoes occur on the day of landfall, or the next day.

Here are the preliminary tornado reports from the 2017 hurricane season. There was a total of 119 reports from 5 tropical cyclones. 2017 ranks 4th in the number of tropical cyclone tornadoes since 1995.

Posted by NOAA NWS Storm Prediction Center on Tuesday, November 7, 2017

The 2004 hurricane season produced the most on record, with 317 tornadoes associated with tropical systems. That year, Hurricane Ivan caused an outbreak of 120 tornadoes, the most ever recorded by a tropical system. The tornadoes were reported over a three day period covering nine states, including 37 in Virginia, 25 in Georgia, and 18 in Florida. 8 people were killed and 17 injured by the tornadoes. The most intense tornado from Ivan hit Remington, VA at F3 strength.  Also in 2004, Hurricane Frances produced 103 tornadoes, number three on the all time list for tornado producing hurricanes. The tornadoes with Frances extended from landfall in Florida all the way north to Maryland. South Carolina saw nearly half the tornadoes from Frances, at 45.

[Storm damage from a tornado in Hurricane Ivan, from NWS Blacksburg, VA]

[Tracks of 50 mesocyclones and some of the tornado vortex signatures identified by radar vorticity images between on September 15 and September 16 from the Tallahassee National Weather Service during Hurricane Ivan.]

Before Hurricane Ivan, the previous record for the most tornadoes in a tropical system was Hurricane Beulah in 1967. The storm spawned a reported 115 tornadoes in southeast Texas during the first several days after its landfall in September, killing five and injuring 32.

2005 set many hurricane records, including the most active hurricane season ever on record with 28 named storms. It come as little surprise that there were several tornadoes reported with tropical systems that season, with a total of 238. Hurricane Rita was responsible for the most tornadoes in 2005 with 101 and is number four on the all time list. A month earlier, Hurricane Katrina produced 59 tornadoes over eight states and five days. That puts the storm as the fifth most active tornado producing tropical cyclone.

[Map of tornado tracks in the warning area Jackson, MS National Weather Service from Hurricane Rita in September 2005. More than half of the Rita tornadoes were in this area (55).]

No EF5 tornadoes have ever been reported from a tropical cyclone, but two F4 tornadoes have been documented from hurricanes moving inland. On Sept. 10, 1961, Hurricane Carla spawned an F4 tornado that killed eight people in Galveston, Texas. A few years later in on Oct. 3, 1964, an F4 tornado from Hurricane Hilda killed 22 people in LaFourche Parish, La.

One of the tornadoes produced by Hurricane Allen in 1980 did about $100 million damage, in recent dollars, in the Austin, TX, area, the most costly tornado from a tropical system.

Credits: Central Pacific Hurricane Center

For WeatherNation: Meteorologist Mace Michaels

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